No, the black text on blue background is hard to read for everyone, but is overrated by the WCAG’s mediocre heuristic because human visual response to light intensity is not actually logarithmic, but closer to a square root. (Under a particular level of light adaptation; if you allow your eye to adapt for 30 minutes you can move the response curve up or down to see in very dim or very bright conditions.)
It would be better to come up with an estimate of plausible ranges of screen brightnesses and a plausible range of surrounding contexts, and then use a real color appearance model to decide on visual contrast at the extremes of those plausible scenarios.
But whoever designed the WCAG guideline was either not an expert in human vision or perhaps thought that would be too complicated to use a model which more closely approximated perceptual responses.
The WCAG guideline should be seen as nothing more than a loose suggestion, with lots of false negatives. That is, if it says something is illegible, then it should probably be changed. but if it says the contrast is okay, then that should be checked by a better model (or human inspection).
Both white-on-medium-blue and black-on-medium-blue buttons are inaccessible and should be avoided if possible.
It would be better to come up with an estimate of plausible ranges of screen brightnesses and a plausible range of surrounding contexts, and then use a real color appearance model to decide on visual contrast at the extremes of those plausible scenarios.
But whoever designed the WCAG guideline was either not an expert in human vision or perhaps thought that would be too complicated to use a model which more closely approximated perceptual responses.
The WCAG guideline should be seen as nothing more than a loose suggestion, with lots of false negatives. That is, if it says something is illegible, then it should probably be changed. but if it says the contrast is okay, then that should be checked by a better model (or human inspection).
Both white-on-medium-blue and black-on-medium-blue buttons are inaccessible and should be avoided if possible.